Invited to Nazi Germany during World War II, Dr. Asagumo is asked by Hitler to collaborate in researching the new weapon "Big X". Concerned about the possible effects of this weapon, Dr. Asagumo intentionally delays the progress of the research, conspiring with his co-researcher, the devious Dr. Engel. Immediately before Germany is defeated by the Allies, Dr. Asagumo is shot to death by the German army but not before implanting a card inscribed with the secret of Big X into his son, Shigeru. An organization claiming alliance with the Nazis appears, steals the card from Shigeru, who now lives in Tokyo, and completes the Big X project, which is revealed to be a drug that can expand the human body without limitation. Dr. Engel's grandson has joined the Nazi Alliance. Recovering Big X from the enemy, Shigeru's son Akira fearlessly challenges the Nazi Alliance and Hans Engel, who are plotting to conquer the world.